This invention is related to fiber reinforced bandages and containment solutions for high-speed applications as well as for devices and components operating at high internal pressure. This invention is particularly useful when a light weight solution is necessary.
The purpose of this invention is to provide reinforcement of cylindrically shaped components with the goal of substantial mass reduction, reliable operation under load and in the whole operational temperature range. Another purpose of this invention is to facilitate manufacturing of such a reinforcement.
Alternative reinforcement approaches presented in literature and other inventions do not allow achieving the specified combination of these goals.
In U.S. Pat. No. 4,401,729 a ceramic tube precompressed by a steel sleeve is proposed. This approach has a number of disadvantages, such as inconsistency of thermomechanical properties of the sleeve and the base tube, leading to reduction of precompression of the ceramic liner at elevated temperatures. This approach imposes very tight restrictions on the accuracy of both the sleeve and the liner. Tension in the steel sleeve is non-uniform, which means that the sleeve material is not used fully. Since steel is a heavy material, there is no overall mass reduction.
In U.S. Pat. No. 5,125,179 a ceramic tube precompressed by a braided carbon fiber sleeve is proposed. The braided sleeve is kept under pretension during curing, which assures mechanical contact between the sleeve and the liner after the curing. The braided structure would not permit high levels of precompression in the liner due to considerable deflection of fibers from the hoop direction. It is well-known that stiffness of composites drops if there is a deviation between the load direction and direction of fibers. Besides, loading fibers before the polymer is cured would not allow achieving substantial levels of pretension. Fibers contain various defects along their length and these defects generally determine the strength of fibers and eventually of the whole bundle. However when the bundle is impregnated with a polymer matrix and the polymer matrix is fully cured, the load becomes distributed between fibers. Since defects are randomly spread over the length of the fiber, the effect of the defects is greatly diminished. This leads to a considerable increase in strength. The mechanical properties of composite materials are therefore greatly affected by the state of the matrix. In U.S. Pat. No. 5,125,179 the pretension is suggested to be applied before the composite has reached its full strength. This leads to low pretension and inefficient use of the reinforcement as well as incomplete use of the properties of the ceramic liner. So weight reduction would be fairly small.
In U.S. Pat. No. 5,915,937 a premade reinforcing carbon fiber composite bandage with a thermoset matrix is suggested to be slid over a metal liner. This approach is also fairly inefficient at least in providing radial reinforcement, because metal and composite have different elastic limits. High strength carbon fiber composites have maximal deformation up to 1.5% or even higher, while elastic limit, for instance, for steels is typically assumed to be 0.2%. During operation both the liner and the bandage being in contact would essentially share the same deformation. Obviously, only a small fraction of the strength of the bandage would be employed. Low specific contribution of the reinforcement could be compensated by its increased thickness. This would increase the cost, impede cooling of the liner during firing and increase the total weight of the component. The overall mass reduction in this case is unsubstantial.
In a publication “Optimal Design of Cylindrical Steel/Composite Hybrid Structures for Gun Barrel Applications” of John Tierney et al. presented at SAMPE Sym. in 2005, a wet-wound epoxy-based pretensioned carbon fiber reinforcement is suggested for a steel liner. Since pretension is applied at an inappropriate moment, when the strength of the composite is fairly low, the applied pretension is also low and the use of the material of reinforcement is inefficient.
In a publication “Prestressed Carbon Fiber Composite Overwrapped Gun Tube” of A. Littlefield and E. Hyland, November 2006 currently accessible on http://www.dtic.mil/ a carbon fiber with a polyetheretherketone (PEEK) matrix is suggested for the reinforcement of the steel barrel. The carbon fiber tow comes already preimpregnated with the matrix by the supplier. Such tows are typically referred as prepregs. As mentioned in this publication the selected material allows curing in place by effectively welding the prepreg tow to the wound laminate. The process utilized a hot-gas torch and a pressure roller in order to consolidate the prepreg. Winding was performed under tension. However since the matrix was melt, the pretension applied to the tow was low compared to the tension that could be applied to the dry tow. This happens because the fibers get some freedom when matrix is melt leading to increased effect of fiber defects and overall reduction of the strength of the tow. Since pretension was relatively low, the overall mass reduction was moderate.
Thus the most common mistake made in alternative approaches is that the properties of the composite reinforcement and the liner are not used fully. The pretension is typically applied to the composite before the composite has reached its full strength. In this respect even a conventional cold-drawn steel wire would have provided a superior level of specific pretension despite its higher density. A composite tow subjected to pretension in the right moment when its strength is maximal, would have allowed even higher overall weight reduction.